1. Field of the Invention
This invention relates to a printing apparatus for printing an image on a recording medium based on input image data.
2. Description of Related Art
Conventionally known as a printing apparatus is such as employing an electrophotographic process in which images are formed upon fixation of toner on predetermined recording media, and especially such a printing apparatus as achieving a tone-expression has been proposed (see, e.g., Japanese Patent Laid-Open No. H08-230234). In the printing apparatus as achieving the tone-expression, time required to expose one pixel is divided into predetermined time periods to set different exposure amount for each time period. The printing apparatus as described above adopts a driving method in which an exposing section is selectively enabled or disabled for each time period based on image data to correspond tone of the image information upon control of a cumulative exposure amount per pixel. Furthermore, such a driving method has been also proposed for this printing apparatus, that a time period required to correct the cumulative exposure amount is added to e period to facilitate an improvement in a printing quality.
The conventional printing apparatus, however, has a problem with a linear tone reproduction curve even where adopting any driving methods as described above since the cumulative exposure amount is set to a parameter used for modulation of image data.
To explain this problem in detail, time required to expose one pixel is divided into eight time periods, in which each exposure amount for each of eight time periods is set to e[0], e[1], e[2], e[3], e[4], e[5], e[6], and e[7] in order of timing occurrence and the each time period is set as described in following Table 1. It is to be noted that an exposure amount unit E is obtained per an exposure time unit T and thus 256 (0˜255×E) patterns of the cumulative exposure amount are realized in total according to combinations of patterns between enable and disable states during divided eight timings.
TABLE 1Setting Example of Exposure Amount and Exposure TimeExposure Amounte [0]e [1]e [2]e [3]e [4]e [5]e [6]e [7]Exposure TimeT * 20T * 21T * 22T * 23T * 24T * 25T * 26T * 27
With the conventional printing apparatus, an exposed location shifts over time in association with shift of the recording medium even where the exposure time is changed to change the exposure amount linearly as described above. Therefore, such a tone-expression as developed based on the above described setting results in occurrence of domains indicated by alphabets A, B in FIG. 47, at which the image information and the tone reproduction do not correspond linearly but reverse to each other. As described above, the problem with the linear tone reproduction curve cannot be solved in using the conventional printing apparatus.
This invention has been invented in consideration of the above background, and it is an object of the invention to provide a printing apparatus capable of realizing the excellent linear tone reproduction curve.